Although it is desirable that an amplifier used for a radio communications system or the like has the input-output characteristics that the gain and phase will not change, amplitude distortion and phase distortion occur in practice as input power is increased. Therefore, as shown in FIG. 18, the linearity of the amplifier is improved by connecting, before or after the amplifier, a distortion compensation circuit having input-output characteristics that cancel the nonlinearity of the amplifier.
Examples of conventional distortion compensation circuits include the technology described in Patent Document 1 or Non-patent Document 1. By way of an example, a distortion compensation circuit described in Non-patent Document 1 is shown in FIG. 20. As shown in FIG. 20, in a distortion compensation circuit 200, an input-side DC blocking capacitor 204, a diode 205 connected in the forward direction and an output-side DC blocking capacitor 206 are connected in this order in series with a signal path between an input terminal 201 and an output terminal 202. In addition, a capacitor 207 is connected in parallel with the diode 205. Then, one end of a bias circuit made up of a bias resistor 208 and a bias short circuit inductor 209 is connected to the signal path between the input-side DC blocking capacitor 204 and the diode 205, and the other end thereof is connected to a bias terminal 203. Further, one end of a bias short circuit inductor 210 is connected to the signal path between the diode 205 and the output-side DC blocking capacitor 206, and the other end thereof is grounded.
The operation of the distortion compensation circuit 200 is described next. A signal (RF) in the radio frequency band is input to the input terminal 201, passes through the input-side DC blocking capacitor 204 and is input to the diode 205. In addition, a bias voltage is applied to the diode 205 from the bias terminal 203 via the bias resistor 208. At this time, a signal waveform in the radio frequency band is clipped by the diode 205, generating a direct current. This direct current increases with an increase in input power in the radio frequency band. Then, the internal resistance of the diode 205 in the radio frequency band decreases with an increase of the direct current. The capacitor 207 thus provides, in accordance with its capacitance, a greater change in the phase characteristics than in the gain characteristics with respect to the input power.
By the above-described action, the distortion compensation circuit 200 of Non-patent Document 1 has the input-output characteristics that the gain is increased and the phase is delayed with an increase in input power. Accordingly, this circuit is effective for an amplifier having the characteristic that the gain is reduced and the phase is advanced with an increase in input power, as shown in FIG. 19, and the distortion compensation circuit 200 of Non-patent Document 1 is applied to an amplifier using a GaAsFET (Gallium Arsenide Field-Effect Transistor). Similarly, the distortion compensation circuit of Patent Document 1 has the input-output characteristics that the gain is increased and the phase is delayed with an increase in input power. Accordingly, this circuit is effective for an amplifier having the characteristic that the gain is reduced and the phase is advanced with an increase in input power, and the distortion compensation circuit of Patent Document 1 is applied to an amplifier or push-pull amplifier using a LDMOSFET (Laterally Diffused Metal Oxide Semiconductor Field-Effect Transistor).
Patent Document 1: JP2002-368546A
Non-patent Document 1: K. Yamauchi, et. al., “A novel series diode linearizer for mobile radio power amplifier,” Proc. of Int. Microwave Symp., 1996, p. 831-834